Ultrafast Faraday spectroscopy in magnetic semiconductor quantum structures

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Abstract
We use an optical probe of magnetization to explore the evolution of carrier-spin scattering and magnetic dynamics in quantum-confined geometries. II-VI magnetic heterostructures are studied using femtosecond-resolved Faraday rotation and exhibit both phase (∼1 ps) and spin scattering (∼6 ps) in concert with a field-tunable terahertz quantum beating of the total carrier spin. Spin-flip processes experienced by photoexcited carriers as they tunnel through nanometer-thick magnetic barriers produce a magnetic perturbation strongly sensitive to the initial magnetic state and the spin orientation of the carriers. Once these carriers have recombined (∼70 ps), the magnetic ions relax through completely different channels of significantly slower decay rate (100 ps–10 ns). The relaxation characteristics are found to be substantially influenced by exchange coupling between adjacent magnetic ions at low temperatures (T<13 K). These low-dimensional magnetic systems yield a rich array of spin phenomena absent in traditional semiconductor heterostructures.